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sqloracle-databasejoinouter-join

Oracle "(+)" Operator

I am checking some old SQL Statements for the purpose of documenting them and probably enhancing them.

The DBMS is Oracle.

I did not understand a statement which read like this:

select ...
from a,b
where a.id=b.id(+)

I am confused about the (+) operator, and could not get it at any forums... (searching for + within quotes didn't work either).

Anyway, I used 'Explain Plan' of SQLDeveloper and I got an output saying that HASH JOIN, RIGHT OUTER, etc.

Would there be any difference if I remove the (+) operator at the end of the query? Does the database have to satisfy some condition (like having some indexes, etc.) before (+) can be used?

Solution

  • That's Oracle specific notation for an OUTER JOIN, because the ANSI-89 format (using a comma in the FROM clause to separate table references) didn't standardize OUTER joins.

    The query would be re-written in ANSI-92 syntax as:

       SELECT ...
     FROM a
LEFT JOIN b ON b.id = a.id

    This link is pretty good at explaining the difference between JOINs.

    It should also be noted that even though the (+) works, Oracle recommends not using it:

    Oracle recommends that you use the FROM clause OUTER JOIN syntax rather than the Oracle join operator. Outer join queries that use the Oracle join operator (+) are subject to the following rules and restrictions, which do not apply to the FROM clause OUTER JOIN syntax: